Electronic module with high cooling power

ABSTRACT

A structure to improve the cooling of electronic modules of the type including a printed circuit board contained between two covers. The structure extracts, by at least one of the covers, heat produced by at least one component borne by the printed circuit board. According to one feature, the cover responsible for extracting the heat bears at least one device having a high thermal conductivity, making it possible to even out the temperatures of the surface of the cover with which it is in contact. Such a structure may be particularly applied to the electronic modules on board aircraft.

The invention relates to the cooling of modular electronic devices,especially but not exclusively to “on board” devices placed on boardaircraft, or tanks, etc.

“On board” devices must operate under harsh environmental conditions(vibrations, shocks, humidity, accelerations, heat dissipationdifficulty).

These difficult conditions may cause breakdowns. This means that theseelectronic units have to be designed for the purpose of making theirrepair and their maintenance easier.

One of the new aeronautical standards stipulates that they be producedin the form of a modular structure called LRM (Line Replaceable Module),such structures being installed in racks.

A rack may include a large number of electronic modules. This makesrapid maintenance, handling and repair work easy, but tends to make itmore difficult to extract the heat produced by the various componentscontained in the modules. These components, of increasingsophistication, produce increasing amounts of heat. The inventions aimsto solve the problem of extracting this heat.

FIG. 1 shows in perspective a rack 1 containing several electronicmodules. The rack 1 is generally parallelepipedal, with a front face 3for inserting and extracting the electronic modules 2 a to 2 e and fiveother faces 4, 5, 6, 6 a, 7. The face 7 consists of a back plate whichcloses off the rack 1 and carries connectors into which thecorresponding connectors of the modules 2 a to 2 e are plugged.

The bottom and top plates 4, 5 have slideways, for example made ofmetal, respectively 9, 9 a, for guiding the electronic modules 2 a to 2e in the rack 1 and for keeping them therein.

The rack 1 may include openings for air circulation, for the purpose ofcooling the electronic modules 2 a to 2 e. The cooling air (shownsymbolically by arrows labeled 30) is generally injected into the rackvia the bottom plate 4, through openings 32 called air inlet openingslocated between the slideways 9. The cooling air 30 circulates betweenthe modules 2 a to 2 e (and possibly in these modules). The heat-ladenair 33 leaves the rack 1 through outlet openings 34, formed between theslideways 9 a and the top plate 5.

FIG. 2 shows in a simplified manner, in cross section, an example of aconventional electronic module structure, as may be found, for example,in patent GB-A-2 270 207.

The electronic module 2 e comprises two covers, 18, 19, for example madeof aluminum, having a thickness of the order of 1 millimeter. Aluminumis a particularly advantageous material from the standpoint of on boardmass, in that it has both a low density and a very good mechanicalrigidity even with a small thickness; it also has a moderate thermalconductivity, although very much less than that of copper.

A printed circuit board 15 is placed in the space bounded by the twocovers 18, 19. It is gripped around its periphery by the edges of thecovers. The printed circuit board 15 bears the various components of anelectronic circuit, among which at least one component 22 is producing alarge amount of heat in operation (for example a powerfulmicroprocessor).

In the example in FIG. 2, the component 22 has a thickness e1 whichcorresponds approximately to a distance d1 between the inside of thefirst cover 18 and that face (face 23) of the printed circuit whichbears the component. The component 22 is thus in practice directly incontact with the first cover 18 via, for example, a thermal interface ormatching layer 25 which promotes thermal contact and is electricallyinsulating (an elastomer or epoxy resin).

Thus, the covers 18, 19, and particularly the first cover 18 in theexample shown, provide, in addition to their function of protecting theprinted circuit 15 and of electrostatic shielding, a heat sink function.

The heat transmitted to the cover is itself extracted by conduction tothe slideways 9, 9 a of the rack and then to the walls of the rack; theheat is also extracted by convection, thanks to the cooling air 30coming from the abovementioned ventilation. Ventilation inside theelectronic modules 2 a to 2 e may be added, by providing bottom and topopenings 26, 27 in the bottom and top walls 18 a, 18 b and 19 a, 19 b ofthe covers, respectively.

Even when the three means mentioned above are combined, the extractionof heat may be insufficient with current components. One of the aims ofthe present invention is to improve this extraction.

For this purpose, the invention provides an electronic module of thetype intended to operate in a rack, comprising at least one printedcircuit board, at least one protective cover defining a housing for theboard, at least one component producing heat in operation and mountedvia its bottom face on the board, and a thermal link between a top faceof the component and the cover, characterized in that the thermal linkcomprises, interposed between the top face of the component and thecover, a device having a high thermal conductivity, this device havingan area greater than that of the component and having a thermalconductivity greater than that of the cover.

The word “interposed” should be understood to mean that the device is atleast partly placed in series between the component and the cover in thethermal circuit of heat flow from the component to the cover. This evenapplies if the device having a high thermal conductivity also includes apart which extracts the heat directly to the natural air or to theforced circulation air which lies outside the cover. However, in apreferred embodiment, the device having a high thermal conductivity isphysically interposed in its entirety between the component and thecover and is applied over its entire area (which is greater than that ofthe component) against this cover.

The term “device having a high thermal conductivity”, or in short “HTCdevice”, should be understood to mean any device or element having abetter thermal conductivity than that of the material of which a coverresponsible for extracting the heat is made, so as to make it possibleto even out or tend to even out the temperature at all points on asurface of the cover with which this HTC device may be brought intocontact. They may, for example, when the cover is made of aluminum, beelements based on one or more materials whose thermal conductivity isgreater than or equal to that of copper, or else devices which involvechanges of phase of a solid, liquid or gaseous element allowingsubstantial amounts of energy to be transported; some of the latterdevices are known especially as “heat pipes” and may consist of a hollowplate containing a liquid, the cooling relying on the energy consumed bythe liquid-to-gas phase change in a closed circuit in the hollow plate.In this case, it will be understood that one speaks of an equivalentthermal conductivity, corresponding to the heat extraction capability:the equivalent conductivity is that of an imaginary material which,having the dimensions of the device (for example the heat pipe), wouldhave the same heat extraction capability.

The application of an HTC device to a region of a cover makes thisregion better able to conduct heat and, as it were, better able todistribute it to the rest of the cover which, although a poorer thermalconductor, then benefits from a larger area conducive to thisconduction. This results overall in an increase in the thermalconductivity of the cover, in a ratio very much greater than that of theincrease in the mass of the cover resulting from the presence of the HTCdevice.

A better understanding of the invention, and further features andadvantages that it affords, will appear on reading the description whichfollows, given as nonlimiting example with reference to the appendedfigures, among which:

FIG. 1, already described, shows a rack of a known type containingconventional electronic modules;

FIG. 2, already described, shows, in a sectional view, the structure ofan electronic module illustrated in FIG. 1;

FIG. 3 is a sectional view of an electronic module according to theinvention;

FIG. 4 is a sectional view of an electronic module in another embodimentof the invention;

FIG. 5 illustrates, in a sectional view, an embodiment of a devicehaving a high thermal conductivity according to the invention, placed ona cover shown in FIG. 3;

FIG. 6a is a sectional view showing a cover illustrated in FIG. 5, inwhich an opening facing a device having a high thermal conductivity ismade;

FIG. 6b is a sectional view showing the cover and the device having ahigh thermal conductivity illustrated in FIG. 6, but with the latterbeing positioned differently;

FIG. 6c is a sectional view similar to FIG. 6a and furthermoreillustrating that the device having a high thermal conductivity isprovided with cooling fins; and

FIG. 7 is a sectional view of the cover illustrated in FIG. 5, showingan embodiment of the invention in which a device having a high thermalconductivity forms an integral part of the cover.

FIG. 3 shows an electronic module 40 according to the invention, in asectional view similar to that of FIG. 2.

The module 40 comprises a printed circuit board 41 between two covers42, 43, for example made of aluminum, similar to those of FIG. 22.

The board 41 bears components among which are the dissipative componentsC1, C2, C3 distributed here over both faces 45, 46 of the board.

According to the invention, one (or possibly more) devices having a highthermal conductivity, or HTC devices, 50, 50 b are interposed in series(in the direction in which the heat is extracted from the component tothe cover) between the top face of certain components (that faceopposite the bottom face turned toward the board) and the cover.

A material having a relatively good thermal conductivity (although verymuch less than that of the HTC device) may fill the gap between thecomponent and the HTC device 50 or 50 b when the height of the componentand the shape of the casing and of the HTC device allow only the topface of the component to touch the HTC device at the same time as theHTC touches the cover. The materials or products most commonly used toprovide this thermal link are, for example, elastomers filled withheat-conducting particles, resins, greases, adhesives, gels and surfacetreatments. It should be noted that sheets of the phase-change type nowexist which consist of a support film bearing a coating which changesstate at a given temperature, thereby allowing the microscopicirregularities to be filled and the thermal contact to be improved.

The function of the HTC device 50 is especially to even out or at leastgreatly reduce the temperature differences presented by a surface withwhich it is in contact. For this purpose, with the first cover 42 (butalso the second cover 43 in the example) being made from aluminum, theHTC device 50 may, for example, be made of copper. However, the HTCdevice may also be made of any other material having a thermalconductivity greater than that of the material from which the coverbearing it is made; it may be considered that the implementation of thedevice according to the invention becomes greatly advantageous when itsthermal conductivity is greater than or equal to 1.5 times that of thematerial of which this cover is made (or from which the latter is made).

Among materials possessing a thermal conductivity even greater than thatof copper, mention may be made, for example, of materials based onhigh-conductivity graphite, especially pyrolitic graphite, or compositesof the carbon/carbon type, or else materials of the type having astructure formed by a substrate with a coating of diamond. Finally, itshould be noted that the HTC device 50 may also be made from a structureemploying the phase change of a liquid, solid or gaseous element, asalready mentioned above and as will be explained further in thecontinuation of the description.

The HTC device 50 is in the form of a plate whose thickness e5, forexample between 1 and 4 millimeters, is chosen to be greater the higherthe thermal power to be extracted.

The HTC device 50, that is to say the HTC plate 50, is preferablydesigned to have a surface S1 of larger area than that of the surface S2of the component. Tests have given a very satisfactory result with anarea of surface S1 of the HTC plate 50 of about 30 cm² (for an area ofS2 of around 8 cm²) and a thickness e5 of around 1.5 millimeters. Thesedimensions of the HTC plate are given solely by way of indication and donot constitute a limiting example—they may be tailored to the dissipatedpower levels; in addition, it should be noted that a reduction inthickness e5 of the HTC plate may be compensated for by an increase inits area and vice versa.

It is possible to further increase the effectiveness of the heatextraction by giving the area of the surface S1 of the HTC plate 50 anelongate shape and by orienting it so that its largest dimension is inthe direction most favorable to cooling it. Its largest dimension may,for example, be approximately parallel to the flow of cooling air 30, orparallel to the height H of the electronic module if the slideways 9, 9a constitute a cold source, or else it may be oriented differently,especially according to tests and/or a particular configuration.

The HTC plate 50 may be made and mounted directly (as shown in FIG. 3)on the cover 42, in the factory, that is to say in the same industrialstep as that for manufacturing the cover 42. This allows excellentthermal contact to be achieved. If the HTC plate 50 has to be attachedto the cover 42 after the latter has been manufactured, it may beadvantageous to do this by means of a layer referred to as a thermalinterface (not shown) in order to guarantee the quality of the contact.

Of course, it is possible to place, if necessary, several HTC devices50, 50 a, on the same cover 42, 43.

The second and third components C2 and C3 mounted on the second face 46illustrate another version of the invention, in which a single HTCdevice or plate, labeled 50 b, serves to promote the extraction of theheat produced by several components. The differences in height of thecomponents C2 and C3 are compensated for by the fill material interposedbetween the top face of the component and the HTC device 50 b.

For example, the HTC plate 50 b in this configuration has an area ofsurface S5 of around 42 cm², which allows the heat produced by the twocomponents C2, C3 to be extracted with an efficiency comparable to thatdescribed above in regard to component C1.

FIG. 4 shows an electronic module 40 a in the configuration that itwould have if the heat were to be extracted from the bottom face of thecomponents.

FIG. 5 is a sectional view of an HTC device, labeled 50 c, of the typereferred to above as a “heat pipe”, employing the effect of a phasechange in an element, for example a liquid, for example water. The HTCdevice 50 c may be used on either of the covers 42, 42 a, 43. In theexample, it is shown mounted on the first cover 42 (shown partly in FIG.5) at the same position, for example, and with the same function as thatin the case of the HTC device 50 shown in FIG. 3.

The HTC device 50 c is therefore mounted on the inner face 42 i of thefirst cover 42. It is in a form similar to that of the HTC plate 50with, for example, the same length L3 (parallel to the height H of thecover) but, however, with a thickness e5 may be greater, for examplearound 3 millimeters, especially depending on the machining means used.In the nonlimiting example of the description, it has two parallelcommunicating channels 60, 61 which extend parallel to the plane of thecover 42 and which constitute a closed circuit. On these two channels,the first channel 60 is in contact with a hot wall 62 intended toreceive heat delivered by the first thermal link 47 illustrated in FIG.3. The second channel 61 is in itself in contact with a cold second wall63 which is contact with the cover 42.

Under these conditions, a certain quantity of water contained in thefirst channel 60 passes into the vapor phase when it is heated by thefirst wall 62 and it passes in vapor form (shown symbolically by anarrow 64) into the cooler second channel 61, where the vapor condensesand then returns in water form 65 to the first channel 60.

The closed circuit consisting of the two channels or microchannels 60,61 may be easily produced on an industrial scale in copper or any othermaterial having a good thermal conductivity. These two channels may beformed by narrow grooves machined, for example, in copper plates 66, 67separated by an intermediate plate 68 and closed by end pieces 69 a, 69b. Narrow grooves, for example 1 or 2 millimeters, for closing thechannels, allows several such closed circuits to be reproduced, inparallel, in the dimensions allocated to the HTC device 50 c.

The very high heat transfer capacity that phase-change systems such asthe HTC device 50 c possess allows them to create a homogeneous andsubstantially uniform temperature distribution over the entire surfaceof the cover 42 to which they are applied.

The increase in the mass of a cover 42, 42 a, 43 by the presence of anHTC device may be compensated for by making, in this cover, an openingfacing at least one HTC device, as shown in FIG. 6a.

FIG. 6a shows schematically, and partly, a cover such as, for example,the first cover 42, in a sectional view similar to that of FIG. 5. Thecover 42 bears any one of the HTC devices described above, for examplethe first HTC device 50 illustrated in FIG. 3. In the example shown, theHTC device 50 is mounted on the inner face 42 i of the cover 42.According to one feature of the invention, the cover 42 has one (ormore) openings 88 formed opposite the HTC device 50. The opening 88 mayhave dimensions (of which only the length L5 is shown) slightly smallerthan those of the HTC device (of which only the length L3 is shown) sothat the HTC device closes the opening 88. Part of the HTC device is indirect contact, outside the cover, with the external air. However, theHTC device is thermally linked to the cover 42 at least around theperiphery of the opening 88, so that the cover for its part contributesto extraction of the heat from the component. Here again, the surfacearea of the HTC device is greater than that of the component with whichit is in contact (preferably direct contact).

The nonlimiting example in FIG. 6a shows a case in which the HTC device50 is attached to the cover 42, after the latter has been manufactured,and in the example it is fixed by screws 89 to the inner face 42 i ofthis cover.

FIG. 6b is a view of the cover 42 similar to that of FIG. 6a, alsoshowing an opening 88. But in the case of FIG. 6b, the HTC device 50 isattached to the cover 42 from the outside, that is to say it is fixed tothe outer face 42 e of this cover. The HTC device 50 may thus beattached to the cover 42 from the outside or else from the inside (FIG.6a), especially for reasons of ease of removing the cover.

FIG. 6c is a view of the cover 42 which differs from that of FIG. 6asolely by the fact that the HTC device 50 includes cooling fins 90. TheHTC device 50 is fixed to the inner face 42 i of the cover 42 and thatpart S1 a of its surface S1 which opens to the outside, by virtue of theopening 88, is provided with fins 90 which tend to promote its cooling.

FIG. 7 is a sectional view of a cover, for example the cover 42, and itshows another way of placing an HTC device 50. In this version, the HTCdevice 50 is placed in the extension of the plate which forms the coverand it forms an integral part of the cover; the HTC device may either beobtained directly by machining or, for example, be produced separatelyand fastened to the cover by welding, for example after it has beenfitted into an opening (not shown) made in this cover. It will beunderstood that, in this case, the device is considered to be insertedat least partly in series between the component and the cover, via itsperiphery welded to the cover.

Of course, in this latter version of the invention, the HTC device 50may optionally be provided with fins 90 (not shown in FIG. 7),optionally over its entire surface S1.

What is claimed is:
 1. An electronic module configured to operate in arack, comprising: at least one printed circuit board; at least oneprotective cover defining a part of a housing for the a least oneprinted circuit board; at least one component producing heat inoperation and mounted by its bottom face on the at least one printedcircuit board; and a thermal link interposed between a top face of theat least one component and the at least one protective cover, andcomprising a device having a high thermal conductivity, having a firstcontact surface area greater than a second contact surface area of theat least one component by a factor of at least two, and having a thermalconductivity greater than that of a material of the at least oneprotective cover.
 2. The electronic module as claimed in claim 1,wherein the device having a high thermal conductivity comprises a plate.3. The electronic module as claimed in claim 2, wherein the devicehaving a high thermal conductivity includes a hollow heat-pipe plate,operating on a principle of a change of phase, in a closed circuit, of aliquid contained in the hollow heat-pipe plate.
 4. The electronic moduleas claim in claim 1, wherein the device having a high thermalconductivity has a thermal conductivity equal to or greater than 1.5times that of said material of the at least one protective cover.
 5. Theelectronic module as claimed in claim 1, wherein the device having ahigh thermal conductivity has an elongated shape to have a largerdimension, to allow said larger dimension to be oriented favorably forcooling the at least one component.
 6. The electronic module as claimedin claim 1, wherein the at least one protective cover has an openingformed substantially opposite the device having a high thermalconductivity.
 7. The electronic module as claimed in claim 1, whereinthe device having a high thermal conductivity forms an integral part ofthe at least one protective cover.
 8. The electronic module as claimedin claim 1, wherein said factor is about 2.62.
 9. The electronic moduleas claimed in claim 1, wherein said factor is about 3.75.